The present invention relates to a rangeflnder for use in an automatic focusing mechanism in a camera. One known automatic focusing system for use in cameras is an active infrared system. According to the active infrared system, infrared radiation is applied to an object to be photographed, and the reflected infrared image of the object is focused on a position detector such as a PSD (Position Sensitive Device) for detection thereby. The distance of the object from the rangefinder is detected based on the principles of the triangular measuring system using the signal produced by the PSD.
The infrared radiation reflected by the object is focused onto the PSD by a single lens positioned in front of the PSD. The reflected infrared radiation that has passed through the lens is converged on the PSD at a position depending on the distance of the object from the rangefinder. The conventional active infrared system is arranged such that the shorter the distance of the object from the rangefinder, the closer to an end of the PSD the infrared radiation reflected by the object is converged on the PSD. The output signal generated by the PSD becomes lower as the position of the radiation approaches the nonlinear end, because the focused image on the PSD becomes more elliptical toward the nonlinear end of the PSD. Consequently, it has been difficult for the conventional active infrared system to detect the distance of an object from the rangefinder with sufficient accuracy over a wide distance range.